The present invention is directed to an apparatus for the deposition of thin films in a substantially vacuum environment. More particularly, the present invention is directed to an apparatus for ion sputtering of a material from a sputtering target, in which individual ions having kinetic energies of several tens of electron-volts (eV) to several tens of thousands of eV strike the target surface and knock off (sputter) target atoms, and in which the target atoms thus liberated from the target surface are collected (condensed) from the vapor phase onto a substrate in the form of a thin film. Still more particularly, the present invention is directed to an ion sputter deposition apparatus in which the sputtering ions originate from an ion beam source situated some distance away from the sputter target.
That mode of ion sputter deposition is well known in the art as ion beam sputter deposition and has several advantages over other ion sputter deposition modes, e.g., diode sputtering or magnetron sputtering. Chief among the advantages of ion beam sputter deposition is that the pressure of the background gas (mostly the same species of gas from which the ions are formed) can be much lower than in the other ion sputtering modes because the sputtering ions are generated in a highly efficient, independently powered, remote ion source. Such known sources are reviewed in "Technology and applications of broad-beam ion sources used in sputtering--Part 1: Ion source technology," H. R. Kaufman, J. J. Cuomo and J. M. E. Harper, J. Vac. Sci. Technol. 21(3) pp.725-736 (1982). The lower background gas pressure means that target atoms sputtered off of the target can traverse the distance from the target to the substrate substantially unimpeded by collisions with the molecules of the background gas. As a consequence, both the kinetic energy and the directionality of the sputtered atoms can be preserved upon arrival at the growing thin film on the substrate. Numerous benefits to the properties of the deposited thin film can be had by exploiting control of that kinetic energy and directionality of the depositing atom flux.
The three chief disadvantages of ion beam sputter deposition are low film deposition rate, poor thickness uniformity of the deposited film and increased possibilities for contamination of the deposited film, compared with other ion sputter deposition modes. All three disadvantages can be significantly alleviated simultaneously if a sputter target with large surface area, relative to the size of the substrate, is used. Then a large-area, high-current sputtering ion beam can be used to irradiate a large areal extent of the target surface, giving both a higher film deposition rate and more uniform thickness of the deposited film across the area of the substrate. Issues relating to film purity and contamination are dominant aspects of the present invention and are discussed below.
Also, many industrial processes require a deposition of multiple layers of different thin film materials with one ion sputter deposition machine. It is necessary to do so without exposing the substrate to the atmosphere, for example, to avoid oxidation or contamination of one layer before a subsequent layer is deposited. Therefore, the machine cannot simply be opened for the exchange of the sputtering target. Examples of heterogeneous multilayer thin film structures of commercial importance presently include spin-valve giant magneto-resistance (GMR) sensors used in magnetic disk data storage technology and multilayer dielectric (MLD) stacks used in optical technology as interference filters, laser mirrors, wave-division multiplexers, and other devices. To make such multilayers on a substrate without removal from vacuum, it is known in the art to make a target holder with multiple targets, each of which can be brought to the sputtering position, one at a time. However, when very large targets are contemplated, that can result in a bulky, cumbersome target array.
Ion beam sputter deposition of thin films from ion beam sputter targets is limited in the chemical purity of the deposited film by the possibility of the ion beam sputtering atoms into the growing film from sources other than the intended sputter target. Therefore, it is of great advantage to have the largest possible areal extent of the sputter target surface. Ideally, the sputter target would intercept all of the sputtering ions projected by the ion beam source. In reality, volume contaminant levels of 1:10.sup.3 to 1:10.sup.5 due to stray sputtering ions are sometimes achieved in present ion beam sputtering applications. In other applications, impurity levels on the order of 1:10.sup.7 are necessary. To achieve that, the largest possible ratio of sputter target area to intended sputtering ion beam irradiation area is also desired. Using a small irradiation area to target area ratio, the stray and uncontrolled portions of an ion beam, which can otherwise strike materials off of the target, will strike the target and not contaminate the growing film with off-target-sputtered atoms.
Another source of chemical impurity for multi-target arrays is cross-contamination of the inactive targets by sputtered material from the active target. Various shielding techniques to prevent such cross-contamination are known in the prior art, as will be described below.
A final aspect of the background technology of ion beam sputter deposition relates to the surface condition which develops on the sputter target. Typically, in the state of the art, the ion beam is incident upon the target not only at a fixed polar angle with respect to the surface normal but also at a fixed azimuthal angle. On a typical, fine-grained (0.1 to 2.0 mm) polycrystalline sputter target, sputtering at those fixed angles results in a slow (hours) evolution of increasingly severe surface roughening. Moreover, because of preferential sputtering of certain crystal planes at certain angles over other crystal planes at other angles, the surface roughing which results often takes the form of "shark-skin" texture, i.e., sharp-pointed surface protrusions which are aimed substantially in the direction of the sputtering ion beam. The time-development of directional surface structures on the target must necessarily affect both the total sputtered atom flux escaping from the target surface and the angular distribution of that sputtered flux. The total escaping sputtered flux affects the film deposition rate, and the angular distribution of that flux affects the uniformity of the deposited film across the area of the substrate.
Various developments in ion sputtering art will now be described briefly. However, it will be apparent that none of them overcomes the above-noted deficiencies of conventional ion sputtering.
U.S. Pat. No. 4,923,585 to Krauss et al teaches an ion beam sputter deposition system comprising an ion neutral source 92 which directs a beam onto each one of plurality of sputtering targets 84, 86 and 88. The sputtering targets 84, 86 and 88 positioned on a rotating carousel 82 which sequentially positions each of the sputter targets 84, 86 and 88 in alignment with the particle beam from the ion source 92. A shutter 106 isolates the deposition substrate 94 from the sputtering targets when the sputtering targets are to be cleaned. An apertured mask 108 allows selective deposition to provide electrical connections or the like. There is no teaching to shield the inactive targets from the sputtered flux from the active target (FIG. 5, col. 6, lines 63-68; col. 7, lines 1-10, line 42-43)
U.S. Pat. No. 5,492,605 to Pinarbasi teaches an ion beam sputter deposition system comprising a vacuum chamber 22 in which an ion beam source 21 is mounted, a target 23 and deposition substrate 31. An ion beam 33 provided by the ion source 21 directed at a multiple targets 23 which are provided on a rotary target support 25. The sputtered atoms 26 emitted by the target material are directed onto a deposition substrate 31 on which is formed a layer of the target material. FIG. 9 shows what might be a loosely fitting shield, not separately numbered, around the support 85 to expose the selected target 91. However, that feature is not described in the specification, and even if it were taken to be a shield, such a shield would not be effective to protect inactive targets from sputtered flux from the selected target 91 (FIGS. 2 and 9; col. 4, lines 45-68).
U.S. Pat. Nos. 4,142,958, 4,793,908 and 5,178,738 disclose an ion beam sputtering apparatus and method for fabricating multi-layer optical films, comprising (as for example in '738) multiple targets positioned on the target holder 8. The target holder 8 is generally in the shape of a cube having four faces on which targets 7a, 7b, 7c, 7d are respectively disposed. The target holder 8 is mounted on a rotary shaft 10 so as to be rotated about its axis in such way as to bring any one of the target to be exposed to the ion beam. The shape of the holder can be planar; thus, the conductor and insulator targets are mounted on the plane and shifted as needed so that ion beams are selectively irradiated to the conductor or the insulators. There is no teaching of shielding of the targets (FIG. 1, col. 4, lines 6-35 in '738).
U.S. Pat. No. 4,560,577 to Mirtich et al teaches a sputter target 62 compound of two portions mounted back-to-back, and includes a metal portion 64 and a polymer portion 66. There is no teaching of shielding (FIG. 4, col. 4, lines 10-30).
The above and other references teach various products that can be made through sputtering processes as well as the basics of sputtering, such as the gas environment and the voltage. Therefore, such matters will not be described in detail here. The disclosures of the above-cited references are hereby incorporated by reference in their entirety into the present disclosure.